This invention relates generally to power driven tools, and more specifically to a power driven tool for tightening or loosening fasteners including an impact drive having an oversized hammer clutch.
Power driven tools for tightening or loosening fasteners (e.g., nuts and bolts) are known, and power driven tools incorporating impact drives that can intermittently provide increased torque for tightening or loosening fasteners are common.
An impact wrench incorporating a ratchet head is disclosed in co-owned U.S. Pat. No. 4,821,611, which is incorporated by reference. A pneumatic motor rotates a clutch case that coaxially houses an impact drive. Under normal operation, a cam ball in the clutch case engages a finger of an impact clutch and rotates the clutch with an output shaft for tightening or loosening the fastener. But when frictional resistance of the fastener exceeds a preselected torque output for the tool, the cam ball slides under the impact clutch finger and pushes the clutch axially forward along the output shaft. This action moves a pair of hammers forward into alignment with a corresponding pair of anvils of the output shaft. The hammers simultaneously impact the anvils producing an increased torque in the output shaft, increasing torque applied to the fastener and acting to overcome the frictional resistance of the fastener. Immediately following the impact, the hammers retreat axially rearward and when the cam ball makes one full rotation with the clutch case, the impact sequence repeats if frictional resistance exceeds the preselected torque of the tool.
The clutch case and cam ball generally move at a rate equal to the output speed of the motor, which is relatively fast. Therefore, when the output shaft is unable to turn the fastener, the cam ball repeatedly pushes the impact clutch and hammers axially forward. This action often occurs so rapidly that the hammers impact the anvils before corresponding surfaces are fully aligned, or alternatively the hammers completely miss the anvils and fail to produce any additional torque. Moreover, when the frictional resistance of the fastener exceeds the additional torque produced by the hammers, the cam ball and impact clutch may unnecessarily push the hammers into repeated alignment with the anvils before an operator disengages the motor. This repeated impact and movement can damage components of the impact drive (e.g., the cam ball and impact clutch) or prematurely wear them out.
Co-owned U.S. Pat. No. 7,080,578, which is incorporated by reference, includes a speed reducing mechanism in the power driven impact wrench. This particular design reduces the speed of the motor output and controls the impact rate of the hammers of the impact drive. Although the components of the impact drive are less prone to damage and wear, the speed reducing mechanism requires the use of additional components adding complexity to the tool.
Accordingly, there is a need for a power driven ratchet tool having an impact drive capable of providing adequate damage and wear protection without using a speed reducing mechanism.